This disclosure relates to power supply systems, and in one or more embodiments, to light-emitting diode (LED) dimming control circuitry and methodology that allows a LED driver to increase the maximum pulse-width modulation (PWM) dimming ratio and avoid LED flicker.
Dimming control for light-emitting diodes (LEDs) may be utilised to provide control of LED brightness without changing LED colour. Thereby, a LED current may be pulse-width modulated with a controllable duty cycle to set a desired level of radiant power (i.e., radiant flux) of the LED. A pulse-width modulated (PWM) control signal with a pre-defined period TPWM (cycle time) assumes a high level during an on-time (pulse-width) TON=DPWM·TPWM and a low level during an off-time TOFF=(1−DPWM)·TPWM (or vice versa if using inverted logic levels). The ratio DPWM=TON/TPWM between the on-time TON and the period TPWM is called duty cycle which is a number between 0 and 1 (often expressed as percentage between 0 and 100 percent). A maximum LED current iMAX that defines the maximum “brightness” (precisely the maximum radiant power) of the LED is “chopped” (i.e., alternately switched on and off) according to the PWM control signal. The average LED current iAVG results in iAVG=DPWM·iMAX. Thus the brightness of the LED can be varied, i.e., the LED can be dimmed by adjusting the duty cycle of the PWM control signal.
In practice commonly used current sources for driving the LED are switching converters with output current control since switching converters provide a comparable high degree of efficiency in terms of power losses.
However, when using switching converters the ratio TON/TPWM (duty cycle) can not be reduced down to arbitrarily low values, i.e., the lower bound for the duty cycle is DPWM,min=TON,min/TPWM, whereby the minimum on-time TON,min depends on the clock frequency (switching frequency fCLK) of the switching converter.
For applications that require a brightness control over a large range and down to very low brightness levels the minimum duty cycle has to be made low which requires the LED driver (i.e., the controllable current source, often switching converters with current control) to be capable of responding to a very low duty cycle PWM control signal. However, a random delay can be observed between a rising edge of the PWM control signal (or a falling edge if using inverted logic levels) and a corresponding edge of the actual LED current. This random delay may be due to a random phase relation between the PWM control signal and the clock signal of the switching converter. At very low duty cycles the duration of the random delay may be a significant fraction of the on-time of the PWM control signal which may result in a flickering of the LED that is visible to the human eye. Furthermore, in cases of a rather long duration of the random delay the “remaining” on-time may be too short for the actual LED current to reach its maximum value iMAX which also results in flicker effects and variations in colour.
In practice the minimum on time TON,min has to be eight to ten times the period (cycle time fCLK−1) of the clock signal of the switching converter. However, for certain applications where a large range of brightness control is required it would be desirable to achieve very lower duty cycles in order to allow for brightness control down to very low brightness levels. There is a need for a novel dimming control technique allowing for a very large range of brightness control without illustrating flicker effects.